Technical Field
The present disclosure generally relates to precision transconductance amplifiers for use in electronic circuits.
Description of the Related Art
Some amplifiers used in electrical circuits are voltage amplifiers having an input voltage and a corresponding output voltage that is greater than the input voltage by a gain factor. Other amplifiers used in electrical circuits are current amplifiers having an input current and a corresponding output current that is greater than the input current by a gain factor. Still other amplifiers used in electrical circuits are transconductance amplifiers having an input voltage and a corresponding output current, or transresistance amplifiers having an input current and a corresponding output voltage. In addition to applying a gain, the transconductance and transresistance amplifiers provide a voltage-to-current or current-to-voltage conversion function, respectively.
Such a conversion function can be implemented in several ways. For example, when a resistor is connected between a DC voltage source, V, and ground, a current, I, flows through the resistor. Therefore, the resistor can be thought of as a device that converts a voltage to a current, wherein the output current is proportional to the applied input voltage, according to Ohm's law: I=V/R, or I=GV. When a voltage source connected to a resistor is further connected in series with an amplifier stage, the result is a transconductance amplifier. Like a resistor, a transconductance amplifier also outputs a current that is proportional to its input voltage. The amplifier stage can be, for example, a transistor, an operational amplifier (“op-amp”) or an arrangement of transistors. The resulting transconductance amplifier provides a voltage-controlled current source for use in electronic circuits, as is known in the art.
Transconductance refers to conductance across a direct current (DC) device, that is, the conductance between the input and the output of the device. Conductance, G, is defined as the reciprocal of resistance in a DC circuit: G (mhos)=1/R (Ω). The term “admittance” refers to conductance in an alternating current (AC) circuit, or the reciprocal of impedance: Y (mhos)=1/Z (Ω), as is well known in the art. The AC equivalent of transconductance is then transadmittance.
A Darlington pair is known in the art of circuit design as a pair of bipolar transistors in which the output current of the first transistor is further amplified by the second transistor. A Darlington pair thus behaves as a single transistor with a high current gain, approximately equal to the square of the gain of a single transistor. A disadvantage of a Darlington pair is that it tends to become unstable at high frequencies, so its operational bandwidth is limited.
A current mirror is known in the art of circuit design as an electrical circuit that can be used to provide a bias current to a load. A current mirror can be thought of as a current-controlled current source.